We plan to continue our examination of the secretory process in pancreatic exocrine cells along four main lines. 1) The interaction of pancreatic peptide secretogogues (pancreozymin, caerulein) with their target cell will be examined on a system of isolated exocrine cells obtained by enzymatic dissociation of the gland. Binding parameters of hormones to their receptor sites will be studied and topographic localization of those sites on the cell at the E.M. level will be undertaken using a variety of tagging procedures. For these purposes hormones carrying both radioactive and photo-affinity probes will be synthesized which, after interaction with the cell, can be covalently coupled to the receptor site by photolysis. This should facilitate both receptor site detection and biochemical characterization. 2) We will continue to examine the molecular mechanisms involved in membrane fusion and fission which accompany secretory granule exocytosis in the parotid gland. The role of membrane protein phosphorylation in this process will be examined. 3) A project will be undertaken to follow directly the fate of secretory granule membranes after exocytosis. Using isolated exocrine cells, granule membrane proteins will be tagged with immunologic reagents or radiochemical probes at the end of induced discharge and the subsequent fate of the tagged membranes followed by electron microscopy and radioautography. 4) Improvements on immunocytochemical techniques for detection of intracellular antigens at the E.M. level will be undertaken. The techniques will be applied to the above problems and will also be used to examine the extent of subcellular specialization of handling of individual secretory proteins within individual cells.